If you use DSS between LTE and NR you have to transmit both LTE and NR Resource Block grids in the same frequency and time domain. Since NR supports lean design, reference signals are transmitted only when there is some data to be scheduled on DL. However, there are some 5G signals those are transmitted periodically such as SSB, NZP CSI-RS, CSI-IM, TRS etc. Location of those signals is overlapped with the LTE references signals, hence special MBSFN subframes that are forbidden for LTE transmission will be used to carry 5G broadcast channel and CSI-RS signals. Usually, there’s 3 MBSFN subframes scheduled within 40ms periodicity, so there will be 7.5% less capacity on LTE (7.5% less number of max PRBs you can allocate on LTE) once you deploy Dynamic Spectrum Sharing LTE <-> NR.